3. Quality of Service
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1 3. Quality of Service Usage Applications Learning & Teaching Design User Interfaces Services Content Process ing Security... Documents Synchronization Group Communi cations Systems Databases Programming Media-Server Operating Systems Communications Opt. Memories Quality of Service Networks Basics Computer Architectures Image & Graphics Compression Animation Video Audio 3. Quality of Service (QoS) 3-1
2 Content 3.1 Motivation 3.2 Characteristics of Real-Time / Multimedia Systems 3.3 QoS Definition 3.4 Resources 3.5 Providing QoS 3.6 QoS Architectures 3. Quality of Service (QoS) 3-2
3 3.1 Motivation Kinds of systems we are dealing with are Sending Application System Software CPU Memory System Software CPU Memory Receiving Application System Software Local: Harddisk recording Interactive DVD Computer based training Network Network CPU Memory Distributed Conferencing Video on demand IP-Telephony Basic terminology Resources Realtime Quality of Service What and how much of it do we need, and how do we describe that? 3. Quality of Service (QoS) 3-3
4 Motivation for QoS A QoS model and its implications - QoS specification - QoS calculation - QoS enforcement QoS has different implications in different fields: Operating system / Resource scheduling File system organization Compression Communication system support Media synchronization User Interface and more Quality of Service (QoS) 3-4
5 3.2 Characteristics of Real- Time / Multimedia Systems Real-time System: A system in which the correctness of a computation depends not only on obtaining the right result, but also upon providing the result on time. Real-time Process: A process which delivers the results of the processing in a given time-span. Real-time applications - examples Control of temperature in a chemical plant - driven by interrupts from external devices - these interrupts occur at irregular and unpredictable intervals Example: Control of a flight simulator - execution at periodic intervals - scheduled by a timer service which the application requests from the OS Common characteristics: - internal and external events that occur periodically or spontaneous - correctness also depends on meeting time constraints! 3. Quality of Service (QoS) 3-5
6 Deadlines in Realtime Systems A deadline represents the latest acceptable time to finish an operation, e.g., for the presentation of a processing result hard? soft Tim e Start process Deadline to finish process Hard deadlines: should never be violated result presented too late (after deadline) has no value for the user violation means severe (potentially catastrophic) system failure Example: Nuclear power plant Soft deadlines: deadlines are not missed by much in some cases the deadline may be missed, but not too many deadlines are missed presented result still has some value for the user Example: train/airplane arrival / departure 3. Quality of Service (QoS) 3-6
7 Realtime System - Requirements Primary goal: deterministic behaviour according to specification results in a variety of requirements Mandatory requirements: Predictable (fast) handling of time-critical events Adequate schedulability Stability under overload conditions Desirable requirements: Multi-tasking capabilities Short interrupt latency Fast context switching Control of memory management Proper scheduling Fine-granularity of timer services Rich set of interprocess communication and synchronisation mechanisms 3. Quality of Service (QoS) 3-7
8 Multimedia Systems A new application area for real-time systems with special characteristics: Typically soft real-time and not (that) critical Requirements may often be adapted to ensure proper handling, e.g., scaling of data streams to available bit rates Characteristics Periodic processing Large bandwidth End-to-end guarantees Fault-tolerance Fairness Standardization 3. Quality of Service (QoS) 3-8
9 3.3 QoS - Definition Quality of Service = well-defined and controllable behavior of a system according to quantitatively measurable parameters Layer model User Application MM System File System Local Processing Transport System Different service objects: Media / Streams Tasks Memory areas 3. Quality of Service (QoS) 3-9
10 QoS - Layer Model Examples: both qualitative / quantitative description Perception QoS Tolerable Synchronisation Drift Visual Perceptability Application QoS Media Parameters Media (Transmission) Characteristics System QoS CPU Rate / Usage Available Memory Communication QoS Packet Size / Rate Bandwidth End-to-End Delay Device QoS Seek / Data Transfer Rate Sample Rate / Resolution 3. Quality of Service (QoS) 3-10
11 QoS Parameters Example: Transport System Common parameters concerning the Transport System are: Throughput Delay / Jitter Loss / Reliability Delay Throughput Loss / Reliability But also: Security Cost Stability (Resilience) 3. Quality of Service (QoS) 3-11
12 QoS Parameter Example Delay Maximum end-to-end delay for transmission of one packet Delay jitter = maximum variance of transmission times Throughput Maximum long-term rate = maximum amount of data units transmitted per time interval (e.g.,packets or bytes per second) Maximum burst size Maximum packet size Loss Sensitivity class: ignore / indicate / correct losses Loss rate = maximum number of losses per time interval Loss size = maximum number of consecutively lost packets 3. Quality of Service (QoS) 3-12
13 Service Classes Guaranteed Service Values or intervals of QoS parameters - deterministic (at any time) - statistical (consider a time interval or a certain propability) QoS min <= P <= QoS max Predictable Service consider history - from the very beginning of calculation - in a shifting time window if it was like that in the last..., you can rely on... Best Effort Service no quarantees given 3. Quality of Service (QoS) 3-13
14 QoS Intervals (1) Parameter values result in acceptable regions inacceptable regions of QoS in one-dimensional intervals application s needs: required QoS desired QoS QoS too bad acceptable QoS QoS too good increasing QoS quality Below required QoS level - no useful service Above required QoS level - unnecessary (useless) resource consumption / cost 3. Quality of Service (QoS) 3-14
15 QoS Intervals (2) Also: multidimensional intervals Frame Rate min. QoS- Resolution max. QoS Resolution 30 max. QoS-Frame-Rate. 20 Acceptable Region 10 0 Unaccept able Region 80x40 min. QoS-Frame-Rate. Resolution (Pixel) 640x Quality of Service (QoS) 3-15
16 3.4 Resources Classification By functionality active resources - actively fulfill a certain task - e.g., processor, network adapter passive resources -provide space - e.g., memory, frequency spectrum, file system By availability for concurrent usage exclusive shared By occurence single multiple Common parameter: Capacity - allows quantitative description 3. Quality of Service (QoS) 3-16
17 Resources - Availability Starting point: scarce, but sufficient resources interactive video requirements insufficient resources Window of Scarcity [Anderson et al., 1990] sufficient but scarce resources high-quality audio network file access abundant resources hardware resources in year X Goal Provide best service at the lowest possible cost Conclusion We need resource management in all components of a multimedia system! 3. Quality of Service (QoS) 3-17
18 Relationship Between QoS and Resources Model Processing, using a QoS before processing Data in Resource with certain capacity QoS after processing Data out 3. Quality of Service (QoS) 3-18
19 Architecture CONTROL Resource Monitor Resource Manager Reservation Database Resource Specific Information DATA Resource Scheduler 3. Quality of Service (QoS) 3-19
20 3.5 Providing QoS Resource Management Phases rejection phase 1 (Setup): user s QoS requirements calculation admission control negotiation of QoS guarantees resource reservation QoS guarantees to user phase 2 (Dat a processing): data arrival on streams QoS enforcement by resource scheduling shaping, loss handling, adaptation 3. Quality of Service (QoS) 3-20
21 p h a s e 1 ( S e t u p ) : QoS Provisioning Setup Phase rejection user s QoS requirements calculation admission control negotiation of QoS guarantees resource reservation QoS guarantees to user Definition of required parameters implicitly or explicitly by application or user Distribution and Negotiation Translation between different layers especially if they use different semantics / notations Transformation QoS parameter => Resource requirements Allocation and coordination of resources along path(s) from source(s) to sink(s) 3. Quality of Service (QoS) 3-21
22 QoS Calculation and Negotiation Model User (Caller) User (Callee) Peer-to-Peer-Relationship (Caller-to-Callee) Application (Caller) Application (C allee) Service User Service Provider System (Caller) System (Callee) 3. Quality of Service (QoS) 3-22
23 QoS Negotiation (1) Bilateral peer-to-peer service provider may not modify requested QoS parameters only service user at receiver side may modify (lower) value(s) in the confirmation message Peer-to-Peer Caller Callee Requested QoS-Value Negotiation QoS ave req QoS ave req QoS ave confirm Changed QoS-V Connect Response t 4 Connect Response t 1 Connect Response t 2 Connect Response t 3 Service Provider 3. Quality of Service (QoS) 3-23
24 QoS Negotiation (2) Bilateral layer-to-layer only between adjacent layers - between local service users and providers - between sender and network Unilateral no modification of requested QoS parameters allowed, but just accept or reject receiver may accept QoS parameter although he cannot meet them - example: color TV broadcast Hybrid uses unilateral mode for a certain bilateral layer-to-layer negotiation - example: broadcast/multicast communication ===> heterogeneity of receivers Further: trilateral for information exchange trilateral for a limited target value 3. Quality of Service (QoS) 3-24
25 Admission Control The system checks whether requested resources are and will be available. Especially important for shared resources: CPU network paths buffer space. A simple rule Check whether the sum of the resources already in use and new request(s) is less or equal to the available resource capacity. More specific: check for schedulability availability of buffers (space) bandwidth Note: strong relationship with Pricing / Billing efficient mechanisms will use economic feedback to prevent users from always requesting the maximum 3. Quality of Service (QoS) 3-25
26 Resource Reservation Fundamental concept for the reliable provision of QoS guarantees! pessimistic - results in Guaranteed QoS guaranteed QoS: needs of appl. 1 unused unused needs of appl. 2 reserved for appl. 1 reserved for appl. 2 tim e optimistic - results in Statistical QoS statistical QoS: needs of appl. 1 needs of appl. 2 conflict reserved for appl. 1 reserved for appl. 2 tim e May use monitoring and react on overload conditions (e.g., CPU load 3. Quality of Service (QoS) 3-26
27 Resource Reservation Aspects - Example Example Communication System ===> variety of aspects Reservation Models Sender-initiated Receiver-initiated Explicit vs. Implicit Out-of-Band vs. In-Band Reservation Style Semantics and Notation Heterogeneity and multicast support Reservation Protocols IP V.5: ST-II RSVP (Resource reservation Protocol) 3. Quality of Service (QoS) 3-27
28 3.5.2 QoS Provisioning - Data Processing Phase phase 2 (Dat a processing): data arrival on streams QoS enforcement by resource scheduling shaping, loss handling, adaptation Maintain resource reservations Use: adequate traffic shaping (to ensure characteristics of processed data) Scheduling algorithms feedback and adaption of the streams 3. Quality of Service (QoS) 3-28
29 Shaping Characteristics of Multimedia Traffic: bursty concurrent requests may cause problems though quarantees could be met (e.g., buffer overflow) Basic principle 3. Quality of Service (QoS) 3-29
30 Shaping Leaky Bucket Algorithm supply Data to be transm itted (supply) Data Capacity Leaky Bucket constant Regulation R N constant Bucket Size determines maximum capacity till overflow (drop) and possible delay Other Algorithms Token Bucket Algorithm Token Bucket Algorithm with Leaky Bucket Rate Control 3. Quality of Service (QoS) 3-30
31 Loss Handling Error Detection by means of redundancy / checks / analysis Loss handling algorithms fall into two basic categories: Retransmission Go-back-N retransmission Selective retransmission Using partially error-free streams Prevention Forward Error Correction (FEC) Priority Coding Slack Automatic Repeat Request 3. Quality of Service (QoS) 3-31
32 Adaption - Feedback Control Monitor the load of network and local end-system resources If significant changes occur, take appropriate action to reduce generated load: Explicit communication receiver tells sender to slow down Completely in network on a hop-by-hop basis By feedback from congested network nodes to the sender. Code/Scale Less/More Monitor/Decode Network Variety of possible reactions e.g., layered transmission adaptive degradation of the stream quality Quality of Service (QoS) 3-32
33 3.6 QoS Architectures Examples (communication layer) Heidelberg Transport System (HeiTS) uses ST-II (IPv5) Internet Integrated Services use existing infrastructure, but deploy dedicated handling of flows (streams) in the transfer system Resource Reservation Protocol RSVP to support heterogenous needs Differentiated Service Granularity based on the TOS (Type Of Service) IP Header Field Define service classes, negotiate service level agreements and ensure dedicated treatment of flows that behave as described IPv6 QoS support was an important design criterion from the beginning Dedicated header fields to allow classification / dedicated treatment of flows 3. Quality of Service (QoS) 3-33
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